Relativistic channels are generated in an under-dense plasma by means of subpicosecond laser pulses. The channel extension is characterized via the Thomson-scattered light and the impact of the laser pulse duration and chirp are studied. It is shown that, under certain conditions, the channel extends further when the pulse duration is increased. This is in contrast to many earlier reports, where the channel grows with increasing laser power. Some theoretical arguments are presented to support the experimental findings. The number of electrons, accelerated in the forward direction, and their energies are assessed. In some cases, it is found that the channel divides and the fragments extend in different directions. This raises the question if, and how, the channel formation can be externally manipulated.
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